Fiber optic sensors are increasingly being used as devices for sensing quantities such as temperature, mechanical strain, displacements, vibrations, pressure, acceleration, rotations, or chemical concentrations. In fiber optic sensors, light is sent through an optical fiber and the returning backscattered light is analyzed. Changes in the parameters of the returning light, as compared to the input light signal baseline, may be measured and tracked.
By phase differencing the reflected signal with a reference signal, minute changes can be detected; these relate directly to the event that is causing the laser signals to be reflected. As one example, acoustic pressure waves in the vicinity of a fiber cable will impart micro strains on the fiber. These micro strains are proportional to the acoustic pressure waves, essentially imparting the frequency of the acoustic pressure wave into the back reflected signal; this is generally referred to a modulating a signal. Phase differencing the reflected signal allows the signal to be demodulated and the acoustic pressure wave reconstructed. This technology essentially turns a fiber optic cable into a microphone.
A growing usage application field for this technology is a fiber sensing system for remote downhole monitoring of oil wells. Other application fields include physical security, such as homeland security and border monitoring. The list of existing and potential applications for this new technology is long and continues to grow. Managing noise associated with the fiber has proven difficult. For example, reduction of acoustic signals impinging on the system hardware that contribute to what is termed signal noise floor has been difficult.